Device for determining and/or monitoring a process variable

ABSTRACT

The invention relates to a device for determining and/or monitoring a process variable. The aim of the invention is to provide a cost-effective, user-friendly device for determining and/or monitoring a process variable. The inventive device comprises the following elements: a sensor, a measuring/regulating/control unit which pre-defines at least one event to be determined or monitored, and at least one memory unit which stores data according to the at least one pre-defined event. The sensor, the measuring/regulating/control unit and the memory unit form a compact unit or an independent field appliance.

FIELD OF THE INVENTION

This invention relates to an apparatus for determining and/or monitoringa process variable.

BACKGROUND OF THE INVENTION

Prior-art measuring devices for determining and/or monitoring anarbitrary process variable (e.g., level, pressure, temperature, flowrate) in industrial measurement technology are so designed that only therespective current measured value is stored and made available. Fordiagnostic, error-detection, and predictive-maintenance purposes,however, the provision of the current measured value is very seldomsufficient. For diagnosis and for the detection of incipient faults, itis necessary to record measurement and/or system information over aprolonged period of time, so that it can be retrieved and evaluated whenrequired.

So far it has only become known to connect a so-called data logger tothe measuring device proper in case of need. The data logger is capableof recording the measurement data over a desired period of time forfuture reference. Commonly used data loggers are offered and sold byEndress+Hauser Wetzer GmbH+Co. KG, for example. These data loggers areused, for instance, when a malfunction of the device is presumed. Usingthe measurement and/or system data recorded by the data logger overtime, a fault diagnosis can be carried out. However, before the fault isactually located, much time may pass.

Error detection is particularly difficult if a malfunction occurs onlyintermittently, for instance at irregular intervals. In that case it mayhappen that in the current measuring period, in which the data logger isrecording measurement and/or system data, the malfunction is notdetectable. The measuring device then operates error-free during therecording of the measurement data; it may happen, however, that themalfunction recurs in the near future and, in the extreme case, is notdetectable by a second check via the data logger, either.

Thus, the known analysis with the help of a data logger which isconnected to the measuring system only from time to time involves therisk that despite a great number of checks, an intermittently occurringmalfunction of the measuring device cannot be detected and,consequently, not be corrected. The method employed in the prior art istherefore uneconomical and may even be dangerous in criticalapplications. An example of a critical application is an intermittentlyoccurring fault in an overfill safeguard mounted in a tank which holdschemicals that are injurious to health.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a low-cost,operator-friendly, and reliable apparatus for determining and/ormonitoring a process variable.

This object is attained by an apparatus comprising a sensor, ameasuring/control unit which specifies at least one event to bedetermined or monitored, and at least one memory unit which stores dataas a function of the at least one predetermined event. In particular,the sensor, the measuring/control unit, and the memory unit form acompact unit or a self-contained field device. Since the apparatusaccording to the invention records measurement and system datacontinuously, arbitrary events can be diagnosed. Such an event is, forexample, a temporary or creeping malfunction of the measuring device. Assoon as an intermittent malfunction occurs, it can be detected with highreliability on the basis of the recorded data, and corrected. However,as will be explained later, the event may also be the current ortime-varying representation of a measured value.

In a further development of the apparatus according to the presentinvention, the process variable is a fill level, pressure, flow rate,temperature, density, conductivity, or any other physical or chemicalquantity to be measured. The event to be determined or monitored is,generally speaking, a predetermined time interval or a predeterminedtime scheme. Furthermore, again generally speaking, the event to bedetermined or monitored may be the attainment of a definedmeasured-value condition or the attainment of a defined system or faultcondition.

In a preferred embodiment of the apparatus according to the presentinvention, both the current measurement and/or system information andthe corresponding measurement and/or system information which occurredin a defined previous time range is stored in the memory unit. Thelatter measurement and system information will hereinafter be referredto as “history data”.

To enable the measuring/control unit to exchange data with a remotecontrol station or to communicate with such a station, in a furtherdevelopment of the apparatus according to the present invention, a bussystem is provided. For the data exchange, any of the knowncommunications standards can be used, such as the Profibus PA standardor the Fieldbus Foundation standard. In particular, both the currentdata and the history data are transmitted over the bus system to theremote control station.

In another preferred embodiment of the apparatus according to thepresent invention, the memory unit for the history data is designed as aremovable compact unit. This embodiment makes it possible to use one andthe same memory unit for a multitude of measuring devices. This reducesthe cost of the apparatus according to the invention. The memory unit inthe compact unit may be, for instance an EEPROM, a hard disk, or avolatile memory device.

In a further development of the apparatus according to the presentinvention, the measuring/control unit has an associated interface viawhich an input/output unit or the memory unit for the history data isselectively connectable to the measuring/control unit. This developmentof the invention is particularly advantageous in that a memory unit canbe used in conjunction with a great number of apparatuses fordetermining and/or monitoring a process variable. In a preferredembodiment, the measuring/control unit carries out a fault analysisand/or a cause analysis and/or a prevention analysis using the historydata provided by the memory unit, and makes the results of the analysesavailable to the operating personnel. The analysis data is displayed tothe operating personnel on an on-site screen, for example.

Preferably, the output unit is designed to provide a pixel-orienteddisplay of current measurement data, intermediate results, history data,and/or analysis data. In the case of level measurements using atransit-time technique, for example, the current measurement data isconveyed to the operating personnel in the form of a so-called echocurve. The echo curve embodies the amplitudes of the echo signals as afunction of transit time or distance travelled. Of course, the displayof the current measurement data derived by a transit-time technique alsoincludes the display of a quantity derived from the echo curve. Anexample is a digital envelope. Typical echo curves are shows in FIGS. 6and 7.

An interesting application of the display of history data is thetemporal variation of the level of a medium in a vessel. By means of thehistory data, it is also possible, of course, to detect and tracemeasurement disturbances.

The present invention will now be explained in more detail withreference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a level-measuring device usingthe transit-time technique;

FIG. 2 is a schematic representation of a first embodiment of theapparatus according to the invention;

FIG. 3 is a schematic representation of a second embodiment of theapparatus according to the invention;

FIG. 4 is a schematic representation of a third embodiment of theapparatus according to the invention;

FIG. 5 is a schematic representation of a fourth embodiment of theapparatus according to the invention;

FIG. 6 shows a typical echo curve as is obtained with guided measurementsignals; and

FIG. 7 shows a typical echo curve as is obtained with freely radiatedmeasurement signals.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows schematically a level-measuring device 16. Level-measuringdevice 16 is mounted in an opening 14 in the lid 15 of the container 12.To determine the level of the medium 11 in the container 12,radio-frequency measurement signals are guided along the surface-wavetransmission line 17 into the medium 11. A measurement signal is shownin FIG. 1 as a radio-frequency pulse in stylized form. The measurementsignals are generated in the signal-generating unit 18 and coupledthrough the coupling unit 19 to the surface-wave transmission line 17.The echo signals reflected from the surface 13 of the medium 11 are fedthrough the coupling unit 19 to the measuring/control unit 3. From thesignal transit time and the height of the container 12, themeasuring/control unit 3 calculates, among other things, the level ofthe medium 11 in the container 12.

FIG. 2 shows schematically a first embodiment of the apparatus accordingto the invention. From the data made available by the sensor 2, forexample by the level sensor 16 shown in FIG. 1, the measuring/controlunit 3 determines the current measured values, i.e., the current levelof the medium 11 in the container 12, for example. The respectivecurrent measurement data is stored in the memory unit 4. Over a definedperiod of time, the respective current measurement data is written intoa memory unit for history data 5, the so-called history memory. On thebasis of the history data it is possible, for example, to carry out afault diagnosis or a cause analysis for short- and long-timemalfunctions of the level-measuring device. Furthermore, the historydata may be used for predictive-maintenance purposes.

In the embodiment shown, the measuring/control unit 3 is connected via abus system 6 to a remote control station 7. Via the bus system 6, thesensor 2 or the measuring/control unit 3 and the control station 7communicate with one another. The input/output unit 8 acts as aninterface to the operating personnel: Here, data can be read out, newparameters can be entered, etc.

The history data is retrieved from the history memory 5 via themeasuring/control unit 3. Therefore, in the embodiment shown in FIG. 2,no additional terminals need be provided on the history memory 5. Theterminal may be located at any point of the bus system 6. The embodimentshown in FIG. 2 is suitable for use in hazardous areas, since thecommunication as such is designed for such use. However, certaindisadvantages arise from the fact that in this embodiment, the bussystem 6 is additionally loaded by the communication with the historymemory 5.

If the communication is to be speeded up, i.e., if more data is to betransmitted over the bus system 6 per unit time, a second embodiment ofthe apparatus 1 according to the invention is appropriate, which isshown in FIG. 3. In this embodiment, the history memory 5 communicateswith the control station 7 via a separate bus system 6. Thus, the bussystem (not shown in FIG. 3) between the measuring/control unit 3 andthe control station is not loaded by a superimposed communication withthe history memory 5. A disadvantage of this embodiment is, however,that an additional connector socket must be provided; furthermore,additional access to the sensor 2 is necessary. In addition, theapparatus can only be used in hazardous areas if the second bus system 6is also designed for use in hazardous areas. Faster communication isthus made possible at increased expense.

FIG. 4 shows schematically a third embodiment of the apparatus accordingto the invention, which can be used if on-line evaluation of the data isnot necessary. In this embodiment, the memory unit 5 for history data isdesigned as a removable compact unit 9. This makes it possible to sendrecorded data to any location for purposes of evaluation and/orsubsequent analysis. Another advantage of this embodiment is that thecommunication over the bus system 6 is not loaded by the transfer of thehistory data. It goes without saying that the embodiment of the historymemory 5 as a removable compact unit 9 can also be used in conjunctionwith the other embodiments of the apparatus according to the invention.

A fourth embodiment of the apparatus according to the invention is shownin FIG. 5. Here, the memory unit 5 for history data or an input/outputunit 8, e.g., an on-site display, can be selectively connected to themeasuring/control unit 3 via the interface 10. The advantage of thisembodiment is that no additional connector socket need be provided forthe history memory 5. A minor disadvantage of this embodiment is,however, that simultaneous operation of input/output unit 8 and historymemory 5 is not possible.

As stated above, the apparatus according to the invention offers a greatnumber of advantages, particularly with regard to error detection,detection of incipient errors (predictive maintenance), etc. For thefirst time, however, it also becomes possible to represent measurementdata graphically. In particular, the representation of the echo curveshould be mentioned, which is evaluated if transit-time techniques areused to determine and/or monitor the level of a material in a container.On an on-site display 8 or at a remote control station 7, the operatorcan visually follow the changes of the level in a container 12 byobserving the shift of the useful-echo signal along the x-axis. Thispossibility of visualization will increase the operator's confidence ina technology in which level is detected via the transit time ofradio-frequency measurement signals or of ultrasonic signals.

Of course, the invention also consists in the fact that the currentmeasurement data is displayed or made available at the input/output unit8. The current measurement data is preferably the echo curve.

FIG. 6 shows the typical echo curve of a TDR sensor. The echo curve, asalready explained above, represents the echo amplitudes of a measurementsignal as a function of the distance travelled by the measurement signalalong the surface-wave transmission line 17, or as a function of thecorresponding transit time.

The first peak in the immediate vicinity of the origin of coordinatesrepresents the so-called fiducial launcher. This peak is caused by astep change in impedance and a resulting partial reflection of themeasurement signal at the interface between the coupling unit 19 and thesurface-wave transmission line 17.

The peak that is farthest from the origin is the end-of-line peak, i.e.,the peak representing that portion of the measurement signal which isreflected at the free end of the surface-wave transmission line 17. Thedistinct peak between the fiducial launcher and the end-of-line peakrepresents the useful-echo signal. The useful-echo signal is a measureof the level of the medium 11 in the container 12. As a result of thestep change in impedance between two media—normally, these are air and asolid or liquid material stored in the container 12—a portion of themeasurement signal is reflected. From the transit time or the distancewhich is determined from the separation between a defined starting pointand the peak of the useful-echo signal, the level can be determined.

While the fiducial launcher and the end-of-line peak representsystem-dependent echo signals which exhibit no dependence on therespective level, the location of the useful-echo signal varies with therespective level: At a low level, the useful-echo signal will move inthe direction of the end-of-line peak; as the level rises, theuseful-echo signal will move in the direction of the fiducial launcher.

It goes without saying that in the case of freely radiated measurementsignals, the end-of-line peak in the echo curve does not occur. However,a peak may occur which is caused by the reflection of the measurementsignal from the bottom of the container. In that case, too, the locationof the useful-echo signal varies with the level of the medium 11 in thecontainer 12, of course. The peak of the useful-echo signal willtherefore move along the x-axis, which represents either the time or thedistance travelled. The measurement signals freely radiated via anantenna may be ultrasonic or microwave signals. It is possible, ofcourse, to use level-measuring devices 16 which are based on the pulsetransit time technique or the FM-CW technique.

If the current echo curve is displayed on a pixel-oriented output unit8, a change in level is visually indicated to the operating personnel.In many cases, this will increase the operating personnel's confidencein the measuring device, since a level change is indicated not simply inthe form of a change in a numerical value, but by means of a signalwhich shifts spatially as a function of a varying level.

1. An apparatus for determining and/or monitoring the fill level of amedium in a container using a transit-time technique comprising: asensor; a measuring/control unit which specifies at least one event tobe determined or monitored; at least one memory unit which stores dataas a function of said at least one specified event and an imput/outputunit which provides a pixel-oriented display of a stored and/or currentmeasurement data, wherein: said display of the current and/or storedmeasurement data is the display of the echo curve made available or thedisplay of the current measurement data is the display quantity derivedfrom the echo curve, and said sensor, said measuring/control unit, saidinput/output unit, and said memory unit form a compact unit or aself-contained field device.
 2. An apparatus as claimed in claim 1,wherein: said at least one specified event to be determined or monitoredis one of: a predetermined time interval, and a predetermined timescheme.
 3. An apparatus as claimed in claim 1, wherein: said at leastone specified event to be determined or monitored is one of: theattainment of a defined measure-value condition, and the attainment of adefined system or fault condition.
 4. An apparatus as claimed in claim1, wherein: said memory unit store both the current measurement and/orsystem information and the corresponding measurement and/or systeminformation that occurred within a defined previous time range (→historydata).
 5. An apparatus as claimed in claim 1, further comprising: a bussystem via which said measuring/control unit or said memory unitcommunicates with a remote control station.
 6. An apparatus as claimedin claim 5, wherein: both the current data and the history data aretransmitted over said bus system to the remote control station.
 7. Anapparatus as claimed in claim 1, wherein: said memory unit for thehistory data is designed as a removable compact unit.
 8. An apparatus asclaimed in claim 1, wherein: said measuring/control unit has anassociated interface via which an input/output unit or said memory unitfor the history data is selectively connectable to saidmeasuring/control unit.
 9. An apparatus as claimed in claim 1, wherein:said measuring/control unit carries out a fault analysis and/or a causeanalysis and/or a prevention analysis using the history data madeavailable by said memory unit, and makes the results of the analysesavailable to operating personnel.
 10. An apparatus as claimed in claim1, wherein: said output unit is designed to provide a pixel-orienteddisplay of current measurement data, intermediate results, history data,and/or analysis data.
 11. An apparatus as claimed in claim 10, wherein:said display of the current measurement data is one of: the display ofthe echo curve made available by a measuring device which determines thelevel of a medium in a container using a transit-time technique, and thedisplay of the current measurement data is the display a quantityderived from the echo curve.
 12. An apparatus as claimed in claim 10,wherein: said display of history data is the display of the temporalvariation of the level of a medium in a container.